Urochordates (or tunicates) are an extremely interesting model for deciphering early development genetic networks. Moreover, they are very close to vertebrates, which makes it possible to document which traits (cell types for example) were already present more than 500 million years ago in chordates and from which vertebrate innovations took place. One of the most studied species of urochordates is the cione Ciona intestinalis (C. intestinalis), which is bred and used in a few laboratories. It has been a traditional study model in developmental biology for many years. Nowadays, the sequencing of its genome represents a tremendous advance in our knowledge of the origin of the vertebrate genome.
Ascidians are hermaphrodites. Fertilization occurs in the water and gives rise to a tadpole-shaped larva 2-3 mm long. The morphology of the larva (head and large tapered tail) allows it to move in the water. The tail is the main characteristic of the larva as it contains a dorsal cord called a notochord (an element that allows the ascidians to be classified in the phylum of chordates).
The front of the larva has three adhesive papillae that will allow it to stick on the ground or on any hard substrate where it can begin its metamorphosis, within 12 to 24 hours after its birth.
A few hours after birth, the tail, with its notochord and nervous axis, is resorbed until it disappears completely in the adult individual. The adhesive papillae grow to become a peduncle that lifts the animal’s body. The different organs will then develop. The passage from larva to adult ascidian takes about twenty days.
A large number of databases and websites related to ascidians are listed on this portal: http://www.tunicate-portal.org/
Global databases :
ANISEED (genomics, literature, expression data and anatomy) http://www.aniseed.cnrs.fr/
Anatomy : FABA/FABA2 : https://www.bpni.bio.keio.ac.jp/chordate/faba/1.4/top.html
Regulatory areas : DBTGR : http://dbtgr.hgc.jp
″Ascidian embryos: from the birth of experimental embryology to the analysis of gene regulatory networks″, Lamy C, Lemaire P., M/S : médecine sciences, vol. 24, n°3, Mars 2008
″Le génome de la cione: une plongée aux origines des vertébrés / The genome of ciona: insights into the origin of vertebrates″, Hervé Tostivint et Hubert Vaudry, M/S : médecine sciences, vol. 19, n° 6-7, 2003, p. 660-662